Male Great Tits (Parus major) adjust dear enemy effect expression in different breeding stages

Many territorial animals are less aggressive towards neighbours than to strangers, a phenomenon known as the ‘dear enemy effect’. However, there has been little research on changes in this behaviour in songbirds across the breeding season. In this study, we followed changes in the dear enemy effect during the breeding season in Great Tits, Parus major . We simulated neighbour and stranger intrusions at three breeding stages corresponding to key ecological and social situations, egg-laying, incubation and nestling stages, and examined the territorial songs and physical responses of territory owners. Our results suggest that neighbours are dear enemies in the egg-laying and incubation stages, when territories are stable, but not at the nestling stage when territories become unstable. In addition, physical responses were more effective as warning signals than territorial songs when dear enemy effects were expressed. We conclude that the dear enemy effect is expressed flexibly, rather than in a fixed way, by territorial male Great Tits in our study area.